Not really. You are basically setting up a second subnet in your home:
ADSL line ---> ADSL Router ----> (external NIC) LMCE Core (internal NIC) ----> Gb Switch ----> multiple devices (PCs, MDs, NASs, HA, VoIP telephones)
The Core sits inline between your broadband modem and your network so that 1) all traffic passes through it to the internal network and 2) so that the Core can provide critical services and control the internal network (such as PnP, like LegoGT mentioned)
It is essential for devices that want to interact with LMCE, and irrelevant to devices that don't, they are just on another subnet like any other.
For the sake of a $15 NIC, just build it the way it is designed. Although possible to re-engineer, it requires a lot of advanced networking and computing knowledge, time, headaches, instability and loss of features.
(BTW, I said it is a second network, ie the bit to the right of the Core in my diagram above. For clarity the "first" subnet is the bit between ADSL Router and (external NIC), this is your existing network, I didn't put anything on that network in the diagram because you can move it all to the second network. So the first network, your existing one, becomes simply a patch cable, nothing more. But as LegoGT says you can put things there if you want. Its just better if they are moved so they become available to LMCE)